This invention relates to improvements in motion sensing transducers especially direction finding and ranging hydrophones.
Direction Finding And Ranging (DIFAR) hydrophones have been used in sonobuoys for nearly two decades to track submarines. A DIFAR hydrophone comprises two directional hydrophones, a pressure hydrophone, and electronics with compass. The receiving sensitivity of a directional hydrophone varies as the cosine of the azimuthal arrival angle of the sound; the two directional hydrophones are orthogonal to one another and form what is called a "crossed dipole". The receiving sensitivity of the pressure hydrophone is omnidirectional, i.e., the receiving sensitivity is at most a weak function of arrival angle. Arrival angles are measured from the reference axis, which is a line that coincides with the maximum receiving sensitivity of an arbitrarily chosen directional channel. This directional channel is the cosine channel; the other is the sine channel.
The crossed dipoles of early DIFAR hydrophones were obtained from four, or multiples of four, pressure hydrophones. Deficiencies of this type of DIFAR hydrophone are:
(1) excessive flow noise at low frequencies unless the distance between pressure hydrophones is large,
(2) low acoustic sensitivity at low frequencies unless the distance between pressure hydrophones is large,
(3) poor null depth (poor direction finding) at low frequencies unless the distance between pressure hydrophones is large,
(4) poor phase and sensitivity tracking between directional channels, particularly when the operating ranges of temperature and hydrostatic pressures are large, PA1 (5) high cost, and PA1 (6) difficulties in packaging in a sonobuoy.
Improved versions of DIFAR hydrophones use other technologies, such as orthogonally mounted accelerometers within a pressure vessel. An acoustic pressure wave accelerates the pressure vessel, as it would a water molecule, producing an output in the accelerometers. The outputs of orthogonal accelerometers form the crossed dipole.
It is possible to build an accelerometer-based DIFAR hydrophone with two accelerometers, but superior performance is obtained with four accelerometers mounted symmetrically within the pressure vessel. The outputs of accelerometers at 180.degree. intervals are combined to form the two directional channels. The use of four accelerometers, as compared with two, increases the acoustic sensitivity, and reduces the sensitivity to unwanted influences, e.g. pressure-induced deformations of the pressure vessel.
In the past, the accelerometers, the pressure hydrophone and the electronics were in the form of separate packages, interconnected with electrical wires and a mechanical suspension.
Accelerometer-based DIFAR hydrophones and other modern DIFAR hydrophones address, with varying degrees of success, the aforementioned deficiencies of the earliest DIFAR hydrophones. The present invention improves still further on these points.